1. Field of the Invention
The present invention relates to a process for the manufacture of die cast thin-walled details of metal and a device in a mold adapted thereto.
2. Description of Known Technology
Die cast metal parts are commonly used today. These are used in many different areas where there are high demands for low weight, function and precision. Details obtained through ingot casting need to be extensively processed after casting to achieve the desired precision. Such details also need fairly thick goods to ensure complete filling of the mold cavity since no pressure can be applied. The latter increases the weight of finished work piece weight which is not desirable. Also inner volumes of articles intended to have a certain capacity is reduced. Injection molding of for example aluminum makes it possible to utilize so called rheo-casting which provides higher homogeneity and reduced risk of forming pores in the molded goods. Injection molding of aluminum is performed in the temperature range 670-720° C. High pressures are used to produce thin-walled parts in particular when utilizing rheo-casting. This implies that high demands on the sealing planes in the mold as the melt has low viscosity and easily creeps into the partition plan. Traditionally, it has therefore been considered impossible to design injection molded details with collapsing core as this would greatly reduce the life expectancy of the mold.
One way known in the art is to utilize separate cores for providing undercuts. These separate cores are assembled into the mold prior to molding and is after the molding pried or knocked out of the molded part. There are several problems with this known method.
First of all, the molded part will shrink onto the separate core and the release angle will therefore have to be rather large, typically in excess of 5°. This will of course limit the practical use of making undercuts.
Secondly the cycle time will increase as the mold will have to be assembled before each molding.
Thirdly the separate cores will be damaged due to all the handling especially when the separate cores are knocked out of the molded part. This will affect the sealing of the mold and consequently limit the useful life of the mold itself.
Fourth, the mold temperature is very important for the quality of the molded detail and there is no practical way to temper the separate cores prior to, as well as during the molding cycle.
Fifth, the use of separate cores will also increase the need for post molding treatment as the inevitable dents on sensitive edges of the separate cores, which are intended to form sealing planes in the mold will cause so called burrs and flash.
Sixth, the knocking and prying operation to remove the separate cores from the molded part will increase the risk for damaging not only the separate core but also the part itself. Small cracks or fissures not unlike those related to as metal fatigue can occur which leads to a need for substantial testing of molded parts with serious functions such as certain critical vehicle components.
Seventh, the separate cores need to be fixed in their proper position during the molding which calls for one or more support pins arranged at the opposite side of the mold. These support pins will leave holes in the molded part and these holes will in most cases have to be plugged after molding.
Eight, all of the above listed problems will inevitably lead to a substantial cost increase for the molded articles due to increased handling, longer cycle times, shorter life expectancy of the molds, increased amount of rejected articles, lower average quality of molded parts, increased testing to assure quality and finally increased work effort on each molded part.